How does non-fragility affect glass rooflight specification (in the UK)?

When it comes to the design and performance of glass rooflights, it’s usually aesthetics and thermal performance that are top of the list of specification considerations. However, the performance of the rooflight for safety, as an overall part of the roof, is another important factor.

It is important to note that in the UK there is no legal requirement for rooflights to be non-fragile; it is only relevant if the customer has specified that requirement. However, it would generally be considered good practice to make roofs non-fragile where possible. Outside of the UK, every country has its own rules and regulations.

A risk assessment should be undertaken to control the risks around any roof work, both during construction and for subsequent maintenance activities, with relevant measures being put in place dependent on the roof type. All overhead glass does have to be a safety glass, either toughened, laminated or wired glass. Heat strengthened glass is not a safety glass. This will be discussed later, in relation to which glass type is most appropriate for various locations.

Classification of Non-Fragility for Roofs

Glazed roofs can be classified in one of the following four types:

Class 0: Unrestricted Access; high standard glass designed to be walked upon.

Class 1: Roofs that will be walked upon for occasional cleaning/maintenance; they will support the weight of people and any equipment without the glass breaking.

Class 2: Not designed to be walked upon, but are required to be non-fragile in the event of a maintenance person falling onto the surface. They will support their fall but the glass may be damaged.

Class 3: Roofs considered to be fragile; additional measures and safety considerations are to be specified such as a supporting handrail.

Once the roof class has been defined the correct specification of rooflight for a particular project can be identified.

What standards apply to rooflight specification?

The main standard that applies to rooflight specification is BS5516-2. This gives recommendations for design, properties and maintenance of sloping glass and plastics glazing sheet materials in overhead situations in the envelope and interiors of buildings. It includes those situations where the sloping glazing extends down to floor level.

For glass rooflights the ACR document references CWCT test guidelines, which have been adopted specifically for glass roof lights. It is important to note that not all roof lights have to be CWCT tested.

Within the CWCT guidelines, of particular note are TN66, TN67 and TN92.

TN66 – This Technical Note provides guidance on safety issues relating to access to glass roofs and maintenance of glass roofs which are not accessible by the public but where people carrying out maintenance to the roof, or to other equipment mounted on the roof, may walk, fall or drop objects onto the glazed part of the roof.

TN67 – This Technical Note describes the testing and assessment of glass roofs to establish compliance with the classification system set out in TN66 and described above.

TN92 – This Technical Note defines ‘deemed to satisfy’ criteria and a simplified test method for glass for use in class 2 roofs.

Considerations for specifying the appropriate rooflight

Ultimately, the type of rooflight and its application will define the requirements. Will the rooflight be used for access, ventilation, light or space?

The choice of glass used for the rooflight will depend upon the roof classification required for a particular project (as defined by the roof class, above).

At this stage, the following factors are important to consider:

– Usage

The intended use of the rooflight is an important aspect to consider. In the case of walk on glass, which is outside the scope of TN66 & 67 and has no single specification to cover all potential uses, the glass must be designed to accommodate floor loads in accordance with EN1991-1-1. The glass make up can differ considerably between standard domestic loads in a private home compared to crowd loads in public buildings such as shops, restaurants and museums. Therefore it is critical that the specifier understands completely the intended use of the glass so that proper precautions can be made when calculating the glazed unit.

– Location

The obvious issue with regards to location is access; can people gain access to the glass? Consideration should be given to where the glass is situated where it could be indirectly affected by other activities carried out on the building. If maintenance tasks are going to be carried out above the glazing such as cleaning windows or maintaining guttering or drainage systems, can tools be dropped on to the glass from above? In addition to this, the location of the glass may also determine the size, shape and weight of each glazed section that can feasibly be specified, so that the glass can be manoeuvred into position and installed to acceptable standards.

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